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The HTTP Pragma header is obsolete for HTTP 1.1, and technically only defined for a HTTP request (not response). Refer to https://www.mnot.net/cache_docs/#PRAGMA ,http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.32. It is superseded by the "Cache-Control" directive. See HTTP 1.1 spec at https://tools.ietf.org/html/rfc7234#section-5.4: 'Because the meaning of "Pragma: no-cache" in responses is not specified, it does not provide a reliable replacement for "Cache-Control: no-cache" in them.' Sending a "Pragma: nocache" response header is a prudent backwards compatibility measure for HTTP 1.0 clients. The intended behaviour is for the majority clients as well as any intermediary proxies to ignore this header. Sending an empty Pragma is a known hack for preventing PHP from adding "Pragma: nocache" to responses with started sessions (see http://php.net/session_cache_limiter), since PHP does not allow unsetting existing header() calls.